1. Field of the Invention
This invention relates to matching the impedance presented by a narrow-band resonator filter to typical load and source impedances over a wide frequency range, and with particular application to broadband systems in which narrow-band filters internal to the system are required to provide matched impedances over the broadband frequency range of the system.
2. Background of the Related Art
A filter circuit typically presents an impedance that varies over frequency, with the impedance of the filter reaching an ideal or characteristic impedance in its pass-band. A band-pass circuit reaches its characteristic impedance at its “on-frequency” (fc) in the pass-band, and presents drastically higher or lower impedance in the stop-band. FIG. 1a illustrates the transfer characteristic 10 of a band-pass filter and its fc 12. FIG. 1b illustrates the impedance characteristics 14,16 presented by dual band-pass filter structures over the same frequency range, reaching an ideal impedance ZO 18 at fc 12. FIGS. 2a and 2b both present low-pass filters that are duals of each other and their respective impedance characteristics reflect their dual nature. A topology that is a dual of another topology produces the same output transfer characteristic but typically one is a parallel structure whereas the other is a series structure. Complementary structures have output characteristics that are the inverse of one another.
FIG. 3a illustrates the well-known principle that maximum power is transferred to a load when the load impedance ZL 26 and source impedance ZO 24 match (i.e. they are equal for a resistive impedance, or the complex conjugate if the impedance is complex). Table 30 of FIG. 3a illustrates the voltage and current values for the circuit 30 for when the filter is an open-circuit and a short-circuit (corresponding to the stop-band of the filter) and when the filter is at its characteristic impedance (corresponding to the pass-band). Given that these impedances are complex, the band-pass filter achieves maximum power transfer of the incident signal eO 20 when the impedance of the filter ZL=ZO*, where ZO*=the complex conjugate of ZO. For the sake of simplicity, the table 30 illustrates the case where ZO is resistive only (i.e. ZO=RO). FIG. 3b illustrates the power transfer of the circuit 30 as a function of the resistive impedance of the filter.
FIG. 4a is a conceptual illustration of a filter 42 driven by a source circuit 40 and driving a load circuit 46. FIG. 4b is a conceptual illustration of the transfer characteristic 42c of the filter 42. For most applications, the existence of frequency components in the stop-band of the filter characteristic is not a significant problem. For broadband multichannel systems, however, the presence of significant out-of-band energy can present a real problem as a result of a mismatch in impedance in the stop-band between the filter and either the load or source. The power transfer will be less than optimal due to the mismatch, so out-of-band components having significant energy generated by either the source 40 and load 46 circuits can be reflected back, creating additional distortion in the system. Moreover, if the reflected signals are manifested as increased current or voltage, a source amplifier may clip or otherwise cause non-linear distortion of the signal provided to the filter 42.
FIG. 5 illustrates an application where the impedance presented by a narrow-band filter can be problematic given the drastic difference between the impedance in the pass-band and the stop-band, and the wide frequency range over which the filter must operate. The circuit of FIG. 5 is a structure common to frequency converters such as the one disclosed in the related application entitled “Agile Frequency Converter For Multichannel Systems Using IF-RF Level Exchange For Improved Noise Rejection,” filed May 18, 2000 and which is incorporated herein in its entirety by this reference. This related application discloses a frequency converter for use in a broadband system that employs the circuit of FIG. 5. In such frequency converters, a mixer 56 is used in conjunction with a local oscillator signal 57 to convert an IF signal to an RF signal. As described in the in the related application, generation of the IF signal requires that it be filtered through a band-pass filter 58, and that the mixing process can produce additional unwanted distortion signals in the RF output signal. The output of filter 58 is coupled to ports of the mixer. Because the mixer is a passive balanced device, signals are generated at all ports that may contain various components of the input signals. These “leakage” signals that can appear at the output coupled to filter 58 likely will be out-of-band for the filter 58. Thus, if the impedance of filter 58 as seen at the port of mixer 56 is significantly mismatched to the impedance of mixer 56 in the stop-band of filter 58, a significant percentage of the power of the incident out-of-band leakage signals will be reflected back into the mixer and will show up in the RF output signal. It should be noted that for other applications, narrow-band filtering might be required for the other ports of mixer 56 as well, making impedance matching of the filters even more critical.
One technique commonly used in the art to match the impedance of a filter over an entire range of operation is a diplexor circuit that is conceptually illustrated in FIG. 6a. This circuit provides a structure that is the exact complement 62 of the band-pass filter 60, such that the impedance seen by the input signal is the source impedance over the entire frequency range, as illustrated by FIG. 6b. It is extremely difficult if not impossible, however, to build a structure complementary to tuned resonator filter circuits in accordance with the diplexor structure of FIG. 6a. Thus, this solution works for filters that are not resonators, but does not work for resonator circuits that are advantageously used internally to broadband frequency applications.
Several improved narrow-band tuned resonator filter circuits for broadband applications are disclosed in related applications entitled “Magnetically Coupled Resonators For Achieving Low Cost Band-Pass Filters Having High Selectivity, Low Insertion Loss And Improved Out-Of Band Rejection,” filed Mar. 16, 1998 having U.S. Ser. No. 09/039,988 and Low Cost, Narrow Band-Pass Tuned Resonator Filter Topologies Having High Selectivity, Low Insertion Loss And Improved Out-Of Band Rejection Over Extended Frequency Ranges, filed Sep. 29, 1999 having U.S. Ser. No. 09/408,826, both of which are incorporated herein in their entirety by this reference.
While there have been some attempts to provide better impedance matching between tuned resonator filters and the devices to which they are coupled, no prior art method or apparatus has been able to provide sufficient matching, particularly at or near the on-frequency of the filter, such that frequencies of IF very close to the frequency of LO such as in the context of the mixing process of FIG. 5.
Therefore, those of skill in the art will recognize that there is a need for a method and apparatus by which the characteristic impedance of narrow-band resonator filters can be matched to the load or source impedance of the structures to which they are coupled over a broad range of frequencies, particularly for application to broadband multichannel systems.